1. Field of the Invention
This invention concerns new isocyanurate foams obtained from organic polyisocyanates and organic polyols and having a substantially open-cell structure, a method for producing them, and vacuum insulation layers that make use of them, such as, for example, insulating walls in refrigerators or deep freezers.
2. Discussion of the Background
It is well known that rigid urethane foams and rigid isocyanurate foams can be obtained by causing organic polyisocyanates and organic polyols to react in the presence of blowing agents, and they are widely used as insulating materials. A feature of rigid isocyanurate foams is that, because they have isocyanurate linkages in their polymer structure, they have better flame- and heat-resistance than rigid urethane foams.
Whereas rigid urethane foams are usually obtained by causing organic polyisocyanate and organic polyol to react in the presence of tin catalysts or tertiary amine catalysts so as to bring the NCO/OH equivalent ratio to about 0.9-1.2, rigid isocyanurate foams are obtained by causing their reaction in the presence of trimerization catalysts so as to bring the NCO/OH equivalent ratio to about 3.0 or greater. Generally, at an NCO/OH equivalent ratio of 3.0 or less there is little isocyanurate linkage, and therefore not much improvement can be expected in flame- and heat-resistance, and conversely if the NCO/OH equivalent ratio is too high, the foam becomes brittle and unsuitable for use. In order to obtain good flame resistance, heat resistance, and mechanical properties, the reactions should be made to take place so that the trimer percentage given by the following formula (1) reaches 18-22%. ##EQU1## In formula (1), Ei is the number of isocyanate equivalents, Eh is the number of hydroxyl equivalents, Wi is the weight of isocyanate, and Wh is the weight of polyol.
Also, because of their advantages such as low thermal conductivity, inertness, low boiling point, and nonflammability, chlorofluorocarbons (CFCs) such as trichloromonofluoromethane (CFC-11) have been used as the best blowing agents for producing rigid urethane foams and rigid isocyanurate foams.
Very recently, due to the ozone depletion problem, CFCs are requested to be phased out by 1995. Alternative physical blowing agents include HCFCs (hydrogenated chlorofluorocarbons), such as HCFC-123 (CHCl.sub.2 CF.sub.3) and HCFC-141b (CH.sub.3 CCl.sub.2 F).
Rigid urethane foams and rigid isocyanurate foams manufactured as described above all have a closed cell structure, normally having a closed cell content of 80-95%. This leads to a high degree of heat insulation. However, in heat insulation using such closed cell foam it is inevitable that there will be heat conduction by the blowing agents in gas form that are enclosed inside the closed cells, and over a long period of time the blowing agents will gradually be replaced by air, which has a higher thermal conductivity, thereby lowering the heat insulation performance even further.
It is well known that, theoretically, the vacuum state has the lowest thermal conductivity and yields excellent heat insulation, and proposals have been put forth for vacuum insulating layers in which the core material is an open cell rigid urethane foam. Examples include Japanese patent application Kokai No. 82-133870 and U.S. Pat. No. 4,668,555.
In the former, a closed cell urethane foam is first made in the conventional way. Then, in a vacuum under high temperature and high humidity conditions, the cell walls are broken to produce an open cell structure. This technique requires generating a vacuum over a long period of time to produce open cells, and it is difficult to produce a completely open cell foam.
In the latter technique, blocked open cell rigid urethane foam is produced by using a cell opening material such as calcium stearate, but it is unclear whether this technique can produce open cells within panels by direct pour-in-place process when applied to refrigerators or the like. In U.S. Pat. No. 4,668,555, a vacuum heat insulating layer is obtained by cutting the produced blocked open cell foam into sheets and vacuum-sealing them in a laminate film consisting of metal and plastic. When actually applied to refrigerators, the process is complicated and not practical. A more serious matter is that this patent uses CCl.sub.3 F (trichloromono-fluoromethane) as the blowing agent, but because this substance destroys the ozone layer, it will no longer be used after the year 2000, and thus the method of this patent is undesirable. No other method is known for making open cell rigid foam using other blowing agents.
Moreover, both of the above patents are for rigid urethane foam having an NCO/OH equivalent ratio of 3.0 or less, which can hardly be said to be flame- or heat-resistant. It has thus far not been possible to obtain, with essentially isocyanurate foam, a foam having an open cell structure whose NCO/OH equivalent ratio exceeds 3.0 and that has superior flame- and heat-resistance.